The present technique relates generally to the field of electrical circuit interruption. More specifically, the present technique relates to methods and apparatus for interrupting electrical currents and paths between a source of electrical power and a downstream load.
In order to protect downstream devices (i.e., loads) from excessive and/or undesirable power levels, electrical pathways coupling the device to the power source are traditionally interrupted before the undesirable power reaches the device. Typically, a protection or interruption device, such as a circuit breaker or a fuse, accomplishes this interruption.
In conventional circuit breakers, a moveable member acts as an electrical bridge during normal operating conditions and is pressed against the contacts to provide a good electrical connection between the contacts and the moveable member. However, if a fault condition, such as an over current condition, is detected, the moveable member is driven out of position, breaking the engagement between the moveable member and the contacts and interrupting the current carrying path.
Currently, a variety of actuation mechanisms are employed to drive the moveable member from a conducting position to an interrupted position. For example, certain actuation mechanisms employ electromagnetic relationships to drive the moveable member into the interrupted position. As one example, “slot-motor” circuit breakers (a device of this type is described in U.S. Pat. No. 5,587,861, which issued on Dec. 24, 1996 to Wieloch et al.) employ a lightweight conductive spanner (i.e., moveable member) that is rapidly displaced under the influence of an electromagnetic field generated by a core and winding arrangement. In summary, when a sufficient current is applied to the slot-motor circuit breaker, the electromagnetic forces produced by this current drives the conductive spanner away from the contacts and, resultantly, interrupt the current carrying path. Moreover, the rapid displacement of the spanner causes a significant investment in expanding arcs that effectively extinguish the arc within an intermediary of stacked conductive splitter plates and internally with respect to the circuit breaker. Advantageously, this investment dissipates the electrical current and mitigates the likelihood of undesirable electrical current reaching the downstream load.
However, during normal operations, traditional slot-motor circuit breakers, for example, employ a biasing mechanism that drives the conductive spanner into engagement with the contacts. By way of example, slot-motor circuit breakers often include a coiled spring that drives the conductive spanner into the contacts, to facilitate a good electrical connection between the spanner and the contacts. Indeed, poor engagement between the contacts and the conductive spanner can increase the electrical resistance of the current carrying path and, in turn, generate undesirable levels of heat that can damage the circuit breaker and/or reduce its performance. Accordingly, the coiled spring is configured to provide a force on the spanner to prevent unacceptable levels of resistive heating and to retain the spanner in its starting position.
Unfortunately, the biasing (i.e., opposing) force that acts against the conductive spanner to engage the spanner with the contacts also opposes the electromagnetic force that rapidly separates the conductive spanner from the contacts during a fault condition. In the case of a coiled spring, the biasing force increases as the conductive spanner is driven further away from the contacts. (Fsp=kx; where Fsp is the force applied by the spring; k is the spring constant and x is the distance displaced.) The magnetic force, however, scales as the square of the fault current and decreases as the spanner is displaced from its starting position. Accordingly, to interrupt the current path by moving the spanner a sufficient distance away from its starting position, a relatively large amount of current is needed to produce an electromagnetic force that is sufficient enough to overcome the biasing or opposing force on the conductive spanner. Thus, certain low amperage fault conditions, which are often referred to a “soft-faults” and which can be damaging to the downstream loads, may not produce a sufficient enough electromagnetic force to overcome the biasing force. Resultantly, if the circuit breaker fails to interrupt the current carrying path, the undesirable power passes to the downstream loads, increasing the likelihood of damage to the downstream loads, for instance.
Moreover, certain electrical codes mandate that a circuit breaker be configured to trip (i.e., respond to a fault condition) over a wide range of potential fault currents, such as from the rated current (100 Amps, for example) up to potentially available fault currents as high as 10,000 Amps or more. Therefore, traditional circuit breakers are configured to include at least two interruption mechanisms: a fast acting slot motor for the high fault currents and a more slowly acting bimetal strip that activates an interruption under lower fault current or overload conditions, for example. To provide effective continuity of coverage between these two mechanisms, the slot motor component should interrupt successfully down to 15 times rated current. Accordingly, in a 100 Amp application, the slot motor component must be able to respond to a fault condition of 1.5 kA.
Under these soft fault conditions, the driving magnetic force is reduced by a factor of approximately 45 as compared to a 10,000 Amp hard fault. Thus, in traditional slot-motor based designs, the biasing force exceeds the electromagnetic force produced by a 1.5 kA current once the spanner begins to move significantly away from its starting position, limiting the range of spanner displacement. In other words, the spanner cannot transition to the interrupted configuration before the electromagnetic force falls below the biasing force, even if the electromagnetic force was initially sufficient to overcome the biasing force. This conflict presents a significant barrier to the effective design of circuit breakers that use a magnetically driven hard fault interruption mechanism, such as a slot motor.
There is a need, therefore, for improved methods and techniques for current pathway interruption.